1. Field of the Invention
The present invention relates to coal-oil mixtures (that is, mixtures of coal dispersed in oil) having excellent stability comprising mixtures of pulverized coal and hydrocarbon oil wherein the pulverized coal contained therein does not precipitate or solidify even after long periods of time.
2. Description of the Prior Art
Hitherto, solid coal has been considered inferior as an energy source to liquid fuel oil because of difficulties of transportation, storage, combustion control, and so forth. In order to overcome these defects and to increase the utility of coal as a fuel, various studies have been conducted concerning so-called coal-oil mixtures (hereinafter referred to as "COMs"), which is prepared by pulverizing the coal and dispersing it in fuel oil. As published in the First International Symposium on Coal Oil Mixture Combustion, held at Florida in the United States in May, 1978, COM is generally prepared by by mixing a petroleum fuel oil with a pulverized coal in an amount of about 20 to 70% by weight of COM. That is, when the pulverized coal content is less than 20% by weight, a use of coal as energy source is insufficient and when the pulverized coal content is more than 70% by weight, the viscosity of the coal-oil mixture is remarkably increased and the fluidity thereof is reduced. In Japan, studies concerning such so-called colloidal fuel similar to COM, prepared by pulverizing coal and mixing it with oil, had been actively conducted by the KAIGUN NENRYOSHO (Naval Fuelyard), etc., before World War II. However, when pulverized coal having a common particle size for conventional direct combustion facilities was merely mixed with petroleum fuel oil, the coal particles separated by precipitation to form a nonfluid solid layer because of differences in the specific gravity of the coal and oil. In order to prevent this phenomenon, the possibility of continuous agitation has been studied as well as the possibility of reducing the precipitation rate of the coal particles by reducing the particle diameter of all of the coal particles to less than 10.mu.. However, such techniques are expensive and are not preferred for practical use.
On the other hand, studies have also been conducted concerning a method of producing a stabilized or emulsified COM by techniques comprising adding a stabilizing agent, such as a high molecular material or an emulsifying agent, etc., and/or water to the petroleum fuel oil and pulverized coal, in order to prevent precipitation of the pulverized coal. Typical stabilizing agents used for the above-described purpose include, for example, protective colloids such as glue, gelatin, gum arabic or starch, etc. In addition, other materials that can be used include paraffin, sericin, lanolin, vaseline, and analogues thereof, such as wax, beef tallow or wool grease, etc. However, these aforementioned stabilizing agents are not absolutely effective, although they do exhibit some degree of stabilizing function.
Other materials that have been proposed include metal soaps of aliphatic acids. As the metal of these metal soaps, Al, Mn, Co, Zn, Ca, Na, K, Pb and Mg have been used. As the aliphatic acids, oleic acid, stearic acid and palmitic acid have been used. In addition, many other studies have also been reported. For example, studies have been made of a process for preventing precipitation of coal particles which comprises adding alkali to form a salt of humic acid in coal, and of a process for producing stabilized COM which comprises adding a suitable amount of oil derived from coal, for example, tar oil containing anthracene, naphthalene, phenanthrene or phenol, etc., anthracene oil and creosote oil, etc., to the mixture in order to disperse the coal particles by deflocculation. The general statements of the above-described studies are reported in Sekitan No Yokai To Koshitsu-Nenryo (The dissolution of a coal and the colloidal fuel) written by Yasutaro Miyazaki, 1940.
In recent times, control of production and high prices charged by countries having petroleum oil resources in the Middle and Near East have caused the occurrence of an energy crisis. Taking this opportunity, diversion of energy resources away from oil has been required throughout the world, and an energy demand structure comprising petroleum fuel oil as the principal energy source has had to be reexamined.
Therefore, COM has been noticed again in relationship to the consumption and economy of petroleum oil resources, and the effective use of coal, and many studies and inventions have been proposed.
Among these proposed studies, there are certain inventions concerning processes for producing stabilized COM without using a stabilizing agent, such as: a process which comprises applying an electric field to a fuel oil dispersion system containing pulverlized coal to reduce the precipitation rate of pulverlized coal caused by increasing in viscosity by an electroviscous effect, as described in U.S. Pat. No. 4,202,670; and a process which comprises incorporating about 3% of superpulverized coal having a particle diameter of 5.mu. or less in the pulverized coal in order to prevent contact of large particles and thereby prevent formation of a dense precipitate, as described in Japanese Patent Application (OPI) No. 40808/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") filed by the present inventors. However, in the majority of these studies stabilizing agents for the COMs are used, for example, in Japanese Patent Application (OPI) No. 18604/78, the nonionic surface active agents having the value of HLB (Hydrophile Lipophile Balance) of 17 to 20 and water-soluble organic polymers are used as stabilizing agents for the COMs. Thus, the majority of the conventional studies relates to the stabilizing agents used.
Addition of a large amount of the stabilizing agent generally improves the stability of COMs, but it also causes increases in the cost thereof. Therefore, it has been desired to develop stabilizing agents having a sufficient stabilizing function even if added to a COM in an extremely small amount.
Various kinds of additives, for example, various imidazoline type surface active agents, bisamide compounds, ether amine derivatives, alkylphenol type surface active agents, the above-described metal soaps, metal salts of carboxyl group containing hydrocarbons, polyethylene glycol type nonionic surface active agents, alkylene oxide derivatives having active hydrogen, alkylarylsulfonic acid type anionic surface active agents and salts of dialkylsulfosuccinic acid ester, etc., have been proposed heretofore. However, stabilizing agents having further improved properties have been desired.
The past studies have concentrated attention on the coal particles as a dispersoid, and the development of stabilizing agents therefor, and the characteristics of hydrocarbon fuel oil as a dispersion medium have not been studied so extensively.
In general, hydrocarbons are divided, roughly, into the types of paraffinic, olefinic, naphthenic and aromatic.
The paraffinic hydrocarbons are saturated chain compounds represented by the molecular formula C.sub.n H.sub.2n+2 (wherein n is a positive integer), and include n-paraffins having no branches and isoparaffins having branches. In petroleum light fractions, the content of paraffinic hydrocarbons is comparatively large.
The olefinic hydrocarbons are unsaturated chain hydrocarbons having one or more double bonds represented by, for example, the general formula C.sub.n H.sub.2n (wherein n is a positive integer) in the case of such compounds having one double bond. Such olefins are not present in appreciable amounts in crude petroleum oil. Petroleum products excepting gasoline contain only very small amount of olefins. Hitherto, the petroleum fuel oils used for COMs contain olefins in the very small amount of 1% by volume or less, which does not contribute to the stability of COMs. Further, diolefins and cyclic olefins are present, if at all, only in minute in petroleum products.
The naphthenic hydrocarbons are hydrocarbons having at least one saturated ring in the molecule. The naphthenic hydrocarbons contained in crude oil or petroleum products are typically those in which two or three naphthenic groups are linked-to one another or condensed with aromatic rings or those which have naphthenic rings or condensed rings having paraffin side chains.
Aromatic hydrocarbons have at least one aromatic ring in the molecule. In light petroleum fractions, benzene and mononuclear compounds having side chains on the benzene ring are the main aromatic components. In heavy petroleum fractions, polynuclear condensed aromatic compounds such as binuclear or trinuclear compounds and compounds containing both of a benzene ring and a naphthene ring are contained as the main aromatic components.
The petroleum fuel oils such as crude oil used as a fuel at present, kerosene (JIS K2203-1972), gas oil (JIS K2204-1976), A-type fuel oil (JIS K2205-1960), B-type fuel oil (JIS K2205-1960), and C-type fuel oil (JIS K2205-1960), etc., are composed mainly of paraffinic hydrocarbons, naphthenic hydrocarbons and aromatic hydrocarbons, and contain only very small amounts of olefinic hydrocarbons.
Moreover, petroleum crude oil or petroleum fuel oil generally contains no appreciable amounts of organic oxygen, if contained, 0.05 wt% or less. For example, the organic oxygen compounds are phenols, naphthenic acid, fatty acids, etc. On the contrary it has been reported by the present inventor in Japanese Patent Application (OPI) No. 129008/79 that oxidized oil of the aromatic hydrocarbon fractions having a boiling point of 200.degree. C. or more showed an excellent effect as a stabilizing agent for the COM. On the other hand, in accordance with the present invention, it has been found that the COM having an excellent stability can be obtained when the above-described oxidized oil is used as a stabilizing agent in the hydrocarbon oils containing at least 5% by volume of olefinic hydrocarbons having at least 8 carbon atoms.